Activation of ERK and SAPK cascades (subgroups of the mitogen- activated protein kinases, MAPK) culminates in opposing cellular phenotypes. ERKs (extracellular signal-regulated kinase) function as mediators of growth factor- or phorbol ester-induced cellular proliferation; whereas, SAPKs (stress-activated protein kinase) are involved in regulation of cytokine-induced cellular differentiation, apoptosis or growth arrest. Although small molecular weight GYP-binding proteins have been shown to be a common element upstream from both ERK (ras) and SAPK (rac), the understanding of the physiologically relevant second messengers that control or provide specificity for these discrete signaling pathways is not dear. Sphingolipid metabolites have been shown to influence kinases and phosphatases both in intact cell and cell- free systems. Sphingolipid-derivatives also regulate receptor autophosphorylation and receptor-linked small molecular weight GYP- binding proteins. Sphingomyelin, the major membrane sphingolipid 1 can be hydrolyzed to form ceramide and then deacylated to generate sphingosine. Inflammatory cytokine-induced cellular differentiation or apoptosis has been linked to ceramide generation; whereas, growth factor-mediated Cell proliferation has been associated with sphingosine formation. The role of ceramide and sphingosine to differentially regulate ERK and SAPK cascades and to determine. in part, the cellular phenotype has been therefore hypothesized. It is specifically hypothesized that growth factor receptor-induced proliferation may be mediated by sphingosine-regulated ERK bioactivity while cytokine receptors may signal an inflammatory response and/or growth arrest through a ceramide-regulated SAPK cascade. The role of sphingolipid metabolites to differentially regulate ERK and SAPK cascades will be investigated at three levels in rat glomerular mesangial cells: 1) as cofactors for specific kinases or phosphatases; 2) as regulators for small molecular weight GYP-binding proteins; 3) as modulators of receptor/ligand binding kinetics. These studies will establish sphingomyelin derived second messengers as key mediators of cellular proliferation and differentiation via activation of discrete protein kinase cascades.